The Mu3e experiment to search for Charged Lepton Flavour Violation in µ+e+e+e- Joost Vossebeld Joost Vossebeld µ3e Seminar PPD 1 Charged Lepton Flavour Violation We see flavour violations in the quark and neutrino sectors, but not for charge leptons SM (with m) does allow for CLFV, but is heavily suppressed. Any observation of CLFV is evidence of NP. CLFV appears naturally in NP theories: E.g new interactions coupling to mixed lepton eigenstates, or new particles with lepton number and a mixing matrix. Some hints NP may show up in the lepton sector: muon g-2, lepton universality. Joost Vossebeld µ3e Seminar PPD 2 PDG 2018 Muon decays If charged lepton flavour is not conserved we also expect neutrinoless decays: e eee and N eN The long lifetime and the limited number of very clean SM decay modes make muon decays ideal to look for rare NP! Joost Vossebeld µ3e Seminar PPD 3 Best limits Projected sensitivities (90%CL) e < 4.3x10-13 MEG (PSI) 4x10-14 MEG II (PSI) eee < 1.0x10-12 SINDRUM (PSI) 4x10-15 Mu3e I (PSI) 1x10-16 Mu3e II (PSI) NeN < 7.0x10-13 SINDRUM II (PSI) 6x10-17 Mu2e (FNAL) Aue Au 7x10-15 COMET I (J-PARC) CLVF -decays 6x10-17 COMET II (J-PARC) MEG-II at PSI Mu3e at PSI COMET at KEK Mu2e at FNAL New experiment will push e sensitivity by up to four orders of magnitude over the next 5-10 years. Joost Vossebeld µ3e Seminar PPD 4 CLFV -decays PDG 2019, “Tests of Conservation Laws” Belle-II Best limits from Belle and Babar, with improvements from Belle-II expected in coming years. Compared to muons: Shorter lifetime and higher BR limits means tau results are less sensitive to NP. This is not compensated by higher tau mass unless NP has unexpected high power dependence on mass or is generation specific Joost Vossebeld µ3e Seminar PPD 5 Other CLFV searches (PDG 2019): BNL E871 BNL E865 LHCb LHCb ATLAS ATLAS + CMS In general, again, the shorter lifetimes and higher BR limits compared to the muons means the results are less sensitive to (generic) NP. Joost Vossebeld µ3e Seminar PPD 6 CLFV Muon decay channels In all cases muons are back-to-back electron 3 co-planar electrons Muon decay from stopped on a target and and photon Pe = 0, Ee = mµ muonic atom. decay at rest. E = Ee = ½ mµ Monochromatic electron Eobserved = mµ E = m -E -E (no neutrinos!) e µ binding recoil Radiative decay: Radiative decay Non-LFV muon decay in e ( eee); orbit, beam related: Accidental backgrounds: Accidental backgrounds prompt antiprotons, e + radiative e + conversion pions,.. photon or Bhabha pairs Joost Vossebeld µ3e Seminar PPD 7 CLFV Muon decay channels In all cases muons are back-to-back electron 3 co-planar electrons Muon decay from stopped on a target and and photon Pe = 0, Ee = mµ muonic atom. decay at rest. E = Ee = ½ mµ Monochromatic electron Eobserved = mµ E = m -E -E (no neutrinos!) e µ binding recoil Radiative decay: Radiative decay Non-LFV muon decay in e ( eee); orbit, beam related: Accidental backgrounds: Accidental backgrounds prompt antiprotons, e + radiative e + conversion pions,.. photon or Bhabha pairs Joost Vossebeld µ3e Seminar PPD 8 CLFV Muon decay channels Sensitive to loop Sensitive to loop Sensitive to loop diagrams (coupling to */Z, Z’,..) (coupling to */Z, Z’,..) (coupling to ) and tree diagrams and tree diagrams (eeeµ) (qqe) q q Joost Vossebeld µ3e Seminar PPD 9 Update from de Gouvea & Vogel, Prog. in Part. and Nucl. Phys. 71 (2013). Physics reach Highly model dependent. Different channels have varying sensitivity to different NP modes. A comparison is possible with a generic Lagrangian model: CLFV experiments have sensitivity up to several PeV effective scale. « 1, loop diagrams » 1 , tree level dominate diagrams dominate Joost Vossebeld µ3e Seminar PPD 10 The 3e experiment at PSI Search for eee with sensitivity for BR > 10-16 using muon beam lines at PSI. Joost Vossebeld µ3e Seminar PPD 11 Muon production at PSI Start from PSI proton cyclotron 590 MeV protons at up to 2.3 mA Joost Vossebeld µ3e Seminar PPD 12 Target-E and the e5 / Compact Muon Beam Line Protons hit fast spinning carbon target. Muons from pion decays near surface peak around p = 29 MeV. e5/CMBL: e5/combined muon beamline (CMBL) captures muons from target, 8 e/µ separation delivering 108 µ/s in a narrow momentum bite stopped on target. + Effective removal e from Michel or 0 decays near target. Joost Vossebeld µ3e Seminar PPD 13 The detector Mu3e targets BR(+ e+e+e ) with a sensitivity of 10-16. This requires: Equivalent to • a fast detector with high bandwidth read-out to cope with up to record searching for one up to 2x109 muon decays per second. grain of sand on all • excellent timing and tracking to separate the signal from background of the Germany’s processes beaches. Joost Vossebeld µ3e Seminar PPD 14 The µ+e+e+e- search: physics constraints The signal: + e+e+e Etotal = m Michel decays with internal conversion: +e+e+e Irreducible background a part from missing ET Etotal < m Accidental backgrounds: Michel positron(s) & electron or e+e- pair from photon conversion or Bhabha scattering. Electrons not (all) from a common vertex Joost Vossebeld µ3e Seminar PPD 15 The µ+e+e+e- search: physics constraints The signal: + e+e+e Etotal = m Michel decays with internal conversion: +e+e+e Irreducible background a part from missing ET Etotal < m Accidental backgrounds: Michel positron(s) & electron or e+e- pair from photon conversion or Bhabha scattering. Electrons not (all) from a common vertex Joost Vossebeld µ3e Seminar PPD 16 + + + mµ - ETotal distribution e e e Irreducible background: +e+e+e BR (3.4×10-5) is 11 orders of magnitude greater R.M.Djilkibaev, than targeted signal sensitivity for eee, R.V.Konoplich PRD7 (2009) but this drops steeply towards kinematic endpoint. Need: (Etot) < 1 MeV Very challenging to achieve for low energy 9 electrons (Ee < 53 MeV) and up to 2×10 muon decays per second. Joost Vossebeld µ3e Seminar PPD 17 The Mu3e detector (Phase 1) DC beam 108 µ/s (Phase-I), <p> = 28 MeV Muons are stopped in double-cone mylar target (75 µm front, 85 µm back) Joost Vossebeld µ3e Seminar PPD 18 The Mu3e detector (Phase 1) Low mass (0.1% X0) HV-CMOS pixel tracker Critical for vertex position and momentum resolution. Inside 1 T solenoidal field Joost Vossebeld µ3e Seminar PPD 19 The Mu3e detector (Phase 1) Timing layers: Scintillating fibres: (t) <500 ps Scintillator tiles: (t) <100 ps Joost Vossebeld µ3e Seminar PPD 20 The Mu3e detector (Phase 1) UK contribution Phase 1 detector: • Clock and reset distribution system (UCL) • Outer and recurl pixel layers (Bristol, Liverpool, Oxford) Joost Vossebeld µ3e Seminar PPD 21 Tracking concept At pe < 53 MeV multiple coulomb scattering dominates track errors. 휃푀푆~ 푋 푋0 (reduce material, 0.1% X0 per Silicon layer) 휎(푝) 휃 ~ 푀푆 (for small ) 푝 Ω But, for very large lever arm ( ~ ) first order effects of 휃푀푆 cancel out. Layer radii are optimised for momentum resolution and acceptance. Joost Vossebeld µ3e Seminar PPD 22 I. Peric, NIMA 650 pp. 158-162, 2011 HV-MAPS sensors Adaptation from CMOS-MAPS using high-voltage compliant CMOS processes. • Specific is deep N-well that collects charge and includes analogue and digital circuits. (no parasitic collection) • N-well is biased to > 80 – 200 V giving 10 - 30 µm depletion in bulk. • High signal and fast charge collection, combining compactness of CMOS with performance of hybrid planar silicon sensors. Critical properties for Mu3e: • Sensors can be thinned to 50 µm without signal loss. • Sensors can operate in a high rate environment ((t) < 15 ns) Mu3e is the first PP experiment to employ HV-MAPS in a tracker 3 Mu3e would not be possible without this new technology! (sensitivity ~ (X/X0) Joost Vossebeld µ3e Seminar PPD 23 MuPix HV-MAPS HV-MAPS development in AMS 360 nm and 180 nm HV-CMOS process, now moved to TSI 180 nm process. MuPix8 (shared submission with ATLASPIX, different substrate resistivities) First large area demonstrator: active area: 1×1.6cm2 • 80 x 80 µm2 • Amplifier in pixel, • Comparator and digital logic in periphery. • 3 (or 1) data outputs, 1.25 Gb/s each • Power: ~250 mW/cm2 MuPix10 first full size chip for detector: active area: 2 × 2 cm2 • Optimised pad layout for module construction • Optimisation power distribution • Some rerouting of signal lines to reduce cross talk • Smaller periphery, • 80 200 cm Submitted Nov 2019, received March 2020 (just before lock down) MuPix11, if necessary, submission Q4 2020Joost VossebeldPixel µ3etracker Seminar assemblyPPD in 2021 24 Results from testbeam: MuPix8 Testbeams with MuPix/FEI4 telescope mostly at DESY and PSI 80 cm 60V ATLASPIX1 Eff. ~ 99.8% (t) ~ 14 ns higher over larger range of thresholds Tuning the threshold we think we with 200 cm can improve this to 6-7 ns Joost Vossebeld µ3e Seminar PPD 25 MuPix mechanics In total for Phase-1 2808 HV-MAPS chips will be mounted to 170 high density interconnect flex circuits to produce the inner and outer layers (1.1 m2 of HV-MAPS sensors) interconnect Material budget is critical 50 µm 15 µm kapton (outer layers only) • 50 µm HV-MAPS (~0.05% X0) HV-MAPS • Ultra thin interconnect flex (~0.05% X0) • 15 µm kapton v-fold strengthening spines (also He-channel) Resulting in approximately 0.1% X0 per tracking layer Joost Vossebeld µ3e Seminar PPD 26 Gaseous Helium cooling ~4.5 kW power dissipation in very low mass structure.